Bucket for next-to-the-last stage of a turbine

ABSTRACT

A next-to-the-last stage steam turbine bucket has a profile according to the table set forth in the specification. The buckets also include continuous couplings at the bucket mid-point including nubs on each bucket projecting from opposite sides thereof in generally circumferentially extending directions. A sleeve is disposed between each pair of adjacent buckets and has open opposite ends for receiving the nubs of the adjacent buckets. The cross-sections of the nubs and open ends of the sleeve are generally complementary and non-circular to prevent sleeve rotation during turbine operation. The buckets have a continuous cover including cover elements having tenons projecting from opposite sides for reception in corresponding openings in the tips of adjacent buckets.

TECHNICAL FIELD

The present invention relates to turbines, for example, steam turbines,and particularly relates to next-to-last stage turbine buckets havingimproved aerodynamic and mechanical properties.

BACKGROUND

Next-to-last stage buckets for turbines are typically connected to oneanother in groups with cover or shroud bands at the tip and a loose tiewire at about the mid-point of the buckets. Unfortunately, under certainconditions, grouped bucket designs such as these can be stimulated bydynamic steam forces and vibrate at the natural frequencies of thegrouped buckets and cover assembly. If the vibration is sufficientlylarge, fatigue damage to the bucket material can occur and lead to crackinitiation and eventual bucket failure. Further, the loose tie wireconnection requires a hole or opening in each bucket which can lead tohigh centrifugal stresses at the hole and greater susceptibility tostress corrosion or fatigue cracking at the hole.

The grouped bucket design also results in gaps between the ends ofadjacent bucket cover groups. These gaps permit steam leakage at the tipbetween cover groups and can reduce the thermodynamic efficiency of thenext-to-the-last stage.

DISCLOSURE OF THE INVENTION

In accordance with the present invention, there is provided a new andimproved next-to-the-last stage bucket for turbines, particularly steamturbines, for use in new turbines as well as replacement buckets foroperating turbines. The present invention incorporates improvedaerodynamic design manifested in a particular bucket profile andcontinuous coupling of the buckets at their tips and near the mid-pointof the buckets' active length to reduce vibratory response and improvemechanical reliability. The buckets are connected at the tip with sideentry covers having a single radially outward-extending sealing rib onthe surface of each bucket tip and cover to reduce steam leakage overthe tip and improve stage thermodynamic efficiency. Instead of loose tiewires through holes in the buckets adjacent their mid-points, continuousloose sleeve connections are provided. This eliminates any need for tiewire holes. The bucket also is overtwisted to compensate for the untwistdue to centrifugal force to improve thermodynamic efficiency.

Accordingly, it is a primary object of the present invention to providea novel and improved bucket for the next-to-the-last stage of a turbine,particularly a steam turbine, and provide a novel and improved bucketwhich has improved aerodynamic design, lower centrifugal stresses,reduced vibratory response and, hence, improved reliability andcontinuous tip sealing to improve thermodynamic performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a bucket tip and cover assemblyfor tip leakage control in accordance with the present invention;

FIG. 2 is a fragmentary elevational view of a pair of bucketsconstructed in accordance with the present invention;

FIG. 3 is a plan view of the buckets with the end covers removed toillustrate the continuous loose sleeve connection at the mid-portion ofadjacent buckets;

FIG. 4 is a fragmentary end elevational view of the end covers foradjacent buckets;

FIG. 5 is a tangential view of a next-to-the-last stage bucketconstructed in accordance with the present invention and illustratingits aerodynamic profile; and

FIG. 6 is a graph illustrating a representative air foil section of thebucket profile as defined by charts set forth in the followingspecification.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings, particularly to FIG. 1, there isillustrated a bucket according to the present invention, generallydesignated 10, having a root section 12 connected to a finger dovetailsection 14 (FIG. 5), and, in turn, for connection to a rotor wheel W(FIG. 2) of the turbine. Bucket 10 also includes a tip 16, to whichcovers 18 are secured as described hereinafter. Portions of the turbinehousing 20 are illustrated in FIG. 1, as well as the nozzle 22 precedingthe next-to-the-last stage turbine bucket rotor wheel W.

Referring now to FIGS. 2 and 3, the buckets 10 are continuously coupledat an intermediate location, preferably a mid-point, along the bucketsby a loose sleeve connection, generally designated 29. To provide suchcontinuous loose connection without forming an opening or a hole throughthe mid-portion of the bucket, there is provided projections 24 and 26on opposite sides of the buckets 10 and which projections are integralwith the buckets. Each projection includes a projecting integral nub 28and 30, respectively. Each nub 28 and 30 is in a non-circularcross-sectional shape, for example, as shown by nub 28 in FIG. 5. Tocouple the adjacent buckets 10 to one another, sleeves 32 open atopposite ends and receive the aligned projecting nubs 28 and 30 of theadjacent buckets 10. The ends of the sleeve lie in a plane at an angleother than 90° to the axis of the sleeve to enable the sleeves to rockas the relative position of the adjacent nubs change with untwist of thebuckets due to centrifugal force. The non-circular cross-sectional shapeof the nubs 28 and 30 when received in the sleeves 32 and wherein thesleeves 32 have essentially a complementary-shaped interiorcross-section, prevent the sleeves from rotating during turbineoperation. A similar type of mid-bucket coupling is described andillustrated in U.S. Pat. No. 5,267,834, issued Dec. 7, 1993, of commonassignee herewith.

Referring now to FIGS. 2 and 4, the tips 16 of the buckets 10 arecontinuously coupled with side entry covers 18 to provide rigid tiprestraint, structural coupling and damping to minimize bucket vibration.The side entry covers 18 comprise individual covers connecting adjacentbuckets. Each of the tips 16 of the buckets have a pair of openings 36and 38. Each bucket cover 18 consists of a parallelogram-shaped piecehaving parallel sides with a tenon 40 which projects upstream to theadmissions side of the bucket. The opposite side of the cover includes atenon 42 which projects downstream to the discharge side of the bucket.The tenon 40 on the bucket admission side is received in an opening inthe tip 16 of the blade and peened over to provide a rigid connection.The tenon 42, however, on the discharge side has a hole in its end whichpermits the tenon to be flared and results in a loose connection withthe tip of the bucket. It will be appreciated, however, from a review ofFIG. 4, that the covers extend continuously about the entirecircumference of the bucket tips to form a continuous closed cover aboutthe turbine bucket tips. A similar type of cover is also disclosed inthe aforementioned U.S. Pat. No. 5,267,834.

It will be appreciated that the dovetails 14 are received in dovetailgrooves in the wheel W whereby the buckets are secured to the turbinewheel. Additionally, the shape of the bucket is twisted to reduce localstresses due to centrifugal forces and untwist during operation. Thatis, the bucket has been overtwisted to compensate for vane untwist dueto centrifugal force to improve thermodynamic efficiency. Note also thata sealing rib 50 (FIG. 1) extends radially outwardly of the surface ofeach bucket tip 16 and cover 18. The rib 50 reduces steam leakage overthe tip and improves stage thermodynamic efficiency.

Referring now to FIG. 6, there is illustrated a representative bucketsection profile at a predetermined distance H (a representative height Hbeing illustrated in FIG. 5) from a datum line D.L. at the intersectionof the bucket root and the bucket base 52. Each profile section at thatradial distance is defined in X-Y coordinates by adjacent pointsidentified by representative numerals, for example, the illustratednumerals 1 through 12 and which adjacent points are connected one to theother along the arcs of circles having radii R. For example, the arcconnecting points 9 and 10 constitutes a portion of a circle having aradius R at a center 54. Values of the X-Y coordinates and the radii Rfor each bucket section profile taken at specific radial locations orheights H from the datum line D.L. are tabulated in the following TableI, including charts identified as Sections 1 through 15. The chartsidentify the various points along a profile section at the given radialdistance H from the datum line D.L. by their X-Y coordinates and it willbe seen that the charts have anywhere from 10 to 24 representative X-Ycoordinate points, depending upon the profile section height from thedatum line. These values are given in inches and represent actual bucketconfigurations at ambient non-operating conditions. The value for eachradius R provides the length of the radius defining the arc of thecircle between two of the adjacent points identified by the X-Ycoordinates. The sign convention assigns a positive value to the radiusR when the adjacent two points are connected in a clockwise directionand a negative value to the radius R when the two adjacent points areconnected in a counterclockwise direction. By providing X-Y coordinatesfor spaced points about the blade profile at selected radial positionsor heights H from the datum line D.L. and defining the radii R ofcircles connecting adjacent points, the profile of the bucket is definedat each radial position and thus the bucket profile is definedthroughout its entire length. The chart titled Section 1 of Table Irepresents the theoretical profile of the bucket at the datum line D.L.The actual profile at that location includes the fillets in the sectionof the bucket connecting the air foil and the dovetail sections, thefillets fairing the profile bucket into the structural base of thebucket.

                  TABLE I                                                         ______________________________________                                        VANE SECTION COORDINATE DESCRIPTIONS                                          POINT                                                                         NUMBER    X            Y        R                                             ______________________________________                                        SECTION NO. 1                                                                 RADIAL DISTANCE FROM DATUM 0.000                                              1         1.78447      -1.26580 -3.92857                                      2         1.37721      -0.71349 -1.85000                                      3         -1.13559     -0.46896 -3.60911                                      4         -1.68789     -0.99355 0.85000                                       5         -1.74095     -1.05038 0.04000                                       6         -1.80764     -1.01179 0.85000                                       7         -1.77276     -0.90582 3.67891                                       8         -1.08881     0.18593  1.47000                                       9         1.07542      0.20273  2.31289                                       10        1.35284      -0.16051 3.03160                                       11        1.58654      -0.62329 10.95865                                      12        1.81670      -1.25084 0.01788                                       13        1.78447      -1.26580                                               SECTION NO. 2                                                                 RADIAL DISTANCE FROM DATUM 1.500                                              1         1.70199      -1.36919 -4.86292                                      2         1.24789      -0.71662 -1.73536                                      3         -1.09578     -0.40625 -3.44758                                      4         -1.56611     -0.80852 0.85000                                       5         -1.65876     -0.89055 0.04800                                       6         -1.73275     -0.83533 0.85000                                       7         -1.70444     -0.76937 4.48660                                       8         -1.08685     0.18439  1.35000                                       9         0.98397      0.17419  3.18686                                       10        1.48299      -0.66119 10.02953                                      11        1.73486      -1.35457 0.01809                                       12        1.70199      -1.36919                                               SECTION NO. 3                                                                 RADIAL DISTANCE FROM DATUM 3.000                                              1         1.59095      -1.47080 -5.30799                                      2         1.10052      -0.70932 -1.65000                                      3         -1.05018     -0.30050 -4.08306                                      4         -1.45582     -0.57661 0.85000                                       5         -1.51850     -0.62066 0.05850                                       6         -1.60491     -0.55130 0.85000                                       7         -1.55072     -0.43210 3.26645                                       8         -1.08157     0.21601  1.30000                                       9         0.86806      0.16372  2.04153                                       10        1.03531      -0.06773 4.08306                                       11        1.32548      -0.62715 6.12460                                       12        1.42771      -0.88408 12.24920                                      13        1.62470      -1.45666 0.01837                                       14        1.59095      -1.47080                                               SECTION NO. 4                                                                 RADIAL DISTANCE FROM DATUM 4.500                                              1         1.46836      -1.54983 -7.52193                                      2         1.03424      -0.80523 -2.89525                                      3         0.94458      -0.67754 -1.65000                                      4         -0.93172     -0.10672 -3.70000                                      5         -1.33873     -0.27921 0.54996                                       6         -1.37754     -0.29662 0.06494                                       7         -1.46116     -0.21002 0.52910                                       8         -1.40788     -0.11693 2.84665                                       9         -1.03464     0.30786  1.20000                                       10        0.59033      0.31863  1.53150                                       11        0.81042      0.07024  2.71178                                       12        1.05241      -0.34487 7.14461                                       13        1.32913      -1.02217 23.57174                                      14        1.50228      -1.53581 0.01841                                       15        1.46836      -1.54983                                               SECTION NO. 5                                                                 RADIAL DISTANCE FROM DATUM 6.000                                              1         1.34294      -1.62054 -12.38616                                     2         0.94641      -0.88968 -2.89011                                      3         0.63594      -0.45741 -1.59357                                      4         -0.82024     0.07109  -2.89011                                      5         -1.23823     -0.02846 0.26837                                       6         -1.29548     -0.04002 0.05367                                       7         -1.35233     0.02661  0.26837                                       8         -1.29613     0.13512  1.85793                                       9         -0.93179     0.46193  1.10000                                       10        0.54598      0.24239  2.89011                                       11        0.95836      -0.45051 10.32180                                      12        1.21272      -1.12201 0.                                            13        1.37714      -1.60626 0.01857                                       14        1.34294      -1.62054                                               SECTION NO. 6                                                                 RADIAL DISTANCE FROM DATUM 7.500                                              1         1.20935      -1.67171 -12.71423                                     2         0.81784      -0.90728 -3.88267                                      3         0.37276      -0.26730 -1.45000                                      4         -0.48323     0.21859  -2.38855                                      5         -1.05880     0.24686  0.39074                                       6         -1.14916     0.25085  0.06152                                       7         - 1.19562    0.33647  0.30764                                       8         -1.11511     0.44505  1.35461                                       9         -0.85643     0.62039  0.95000                                       10        0.24530      0.45822  2.00000                                       11        0.49239      0.16644  3.00000                                       12        0.83685      -0.49039 23.70320                                      13        1.13248      -1.32508 0.                                            14        1.24346      -1.65807 0.01840                                       15        1.20935      -1.67171                                               SECTION NO. 7                                                                 RADIAL DISTANCE FROM DATUM 8.999                                              1         1.07984      -1.70073 -11.67688                                     2         0.67372      -0.86186 -7.92699                                      3         0.27941      -0.20293 -2.21998                                      4         0.13869      -0.01421 -1.05609                                      5         0.02557      0.10227  -1.07487                                      6         -0.14633     0.22614  -1.51841                                      7         -0.37390     0.33401  -2.22949                                      8         -0.69155     0.42718  -3.17886                                      9         -0.96733     0.47367  0.49077                                       10        -0.98381     0.47599  0.09373                                       11        -1.03444     0.63583  0.49077                                       12        -0.96291     0.69322  1.01709                                       13        -0.86149     0.75218  0.78634                                       14        -0.64240     0.82491  0.85253                                       15        -0.40190     0.83251  0.71409                                       16        -0.18936     0.77487  1.03167                                       17        0.07821      0.60158  1.43162                                       18        0.32282      0.31664  2.96152                                       19        0.55030      -0.10247 7.46617                                       20        0.82124      -0.80951 -32.93732                                     21        0.86633      -0.94677 0.                                            22        0.99087      - 1.32207                                                                              0.                                            23        1.11393      -1.68759 0.01829                                       24        1.07984      -1.70073                                               SECTION NO. 8                                                                 RADIAL DISTANCE FROM DATUM 10.499                                             1         0.96505      -1.71272 -13.53744                                     2         0.29872      -0.30065 -8.15240                                      3         0.21421      -0.14930 -1.55343                                      4         -0.20953     0.32743  -2.72577                                      5         -0.77204     0.64825  0.36823                                       6         -0.89983     0.73813  0.07420                                       7         -0.88977     0.84774  0.36976                                       8         -0.71956     0.91811  0.87530                                       9         0.07530      0.61181  1.62344                                       10        0.27165      0.31538  5.19959                                       11        0.54445      -0.31334 13.26790                                      12        0.67686      -0.70253 0.                                            13        0.99917      -1.70035 0.01816                                       14        0.96505      -1.71272                                               SECTION NO 9                                                                  RADIAL DISTANCE FROM DATUM 11.999                                             1         0.86781      -1.72141 -20.31960                                     2         0.55963      -0.94949 -13.28948                                     3         0.28211      -0.33728 -3.37197                                      4         0.05294      0.06353  -2.49469                                      5         -0.19291     0.37540  -3.10902                                      6         -0.48662     0.65367  -4.84913                                      7         -0.66240     0.79297  0.52090                                       8         -0.75100     0.87807  0.09948                                       9         -0.67847     1.04058  0.52090                                       10        -0.47003     1.00686  1.65807                                       11        -0.31357     0.93789  0.93095                                       12        -0.06450     0.75463  1.20329                                       13        0.10809      0.52805  3.20470                                       14        0.31435      0.11150  4.59350                                       15        0.41090      -0.14729 22.57370                                      16        0.59971      -0.72977 0.                                            17        0.90197      -1.70974 0.01806                                       18        0.86781      -1.72141                                               SECTION NO. 10                                                                RADIAL DISTANCE FROM DATUM 13.501                                             1         0.79368      -1.72303 -25.49671                                     2         0.33539      -0.51451 -11.02362                                     3         0.29859      -0.42555 -3.21078                                      4         0.13557      -0.09184 -5.51095                                      5         -0.61623     0.94242  0.23100                                       6         -0.67195     1.04914  0.06271                                       7         -0.61692     1.12507  0.22800                                       8         -0.51909     1.11340  0.94185                                       9         -0.10695     0.82778  1.83191                                       10        0.14032      0.44192  3.96402                                       11        0.25993      0.16395  6.32042                                       12        0.43416      -0.35323 0.                                            13        0.82754      -1.71221 0.01778                                       14        0.79368      -1.72303                                               SECTION NO. 11                                                                RADIAL DISTANCE FROM DATUM 15.001                                             1         0.74325      -1.72355 -19.62688                                     2         0.28383      -0.46661 -5.74502                                      3         -0.15002     0.39114  -14.29570                                     4         -0.52496     0.96830  0.38800                                       5         -0.58609     1.11574  0.06475                                       6         -0.50304     1.18948  0.35900                                       7         -0.39363     1.13298  1.58511                                       8         0.01239      0.67755  2.29099                                       9         0.18184      0.32599  6.63668                                       10        0.37564      -0.26469 0.                                            11        0.77686      -1.71340 0.01756                                       12        0.74325      -1.72355                                               SECTION NO. 12                                                                RADIAL DISTANCE FROM DATUM 16.501                                             1         0.68283      -1.72099 -17.20274                                     2         0.26957      -0.50368 -9.52530                                      3         -0.11590     0.37151  -23.10623                                     4         -0.31559     0.76024  -31.11145                                     5         -0.45903     1.02987  0.39216                                       6         -0.50020     1.14921  0.07490                                       7         -0.39227     1.22859  0.39216                                       8         -0.26901     1.12977  2.28339                                       9         0.09409      0.51823  5.92811                                       10        0.24417      0.09678  10.27896                                      11        0.40454      -0.47008 0.                                            12        0.71629      -1.71181 0.01735                                       13        0.68283      -1.72099                                               SECTION NO. 13                                                                RADIAL DISTANCE FROM DATUM 18.001                                             1         0.64091      -1.71118 -17.28406                                     2         0.20404      -0.38206 -8.64203                                      3         0.00274      0.10935  -27.28406                                     4         -0.42166     1.01588  0.45200                                       5         -0.46546     1.16829  0.07000                                       6         -0.35881     1.23493  0.45200                                       7         -0.23162     1.11616  2.43636                                       8         -0.00574     0.73497  3.07272                                       9         0.11905      0.43177  8.64203                                       10        0.36790      -0.41426 0.                                            11        0.67621      -1.70193 0.01825                                       12        0.64091      -1.71118                                               SECTION NO. 14                                                                RADIAL DISTANCE FROM DATUM 19.501                                             1         0.59863      -1.69873 -19.27374                                     2         0.20716      -0.46908 -24.93439                                     3         -0.32517     0.89876  27.14224                                      4         -0.39138     1.05649  0.38605                                       5         -0.42037     1.18282  0.07373                                       6         -0.30595     1.24842  0.38605                                       7         -0.19000     1.12784  1.63497                                       8         -0.13435     1.02899  3.46094                                       9         0.05944      0.57983  7.42024                                       10        0.20472      0.11653  13.45749                                      11        0.36449      -0.51753 0.                                            12        0.38701      -0.61699 -38.67263                                     13        0.39245      -0.64102 0.                                            14        0.63139      -1.69039 0.01691                                       15        0.59863      -1.69873                                               SECTION NO. 15                                                                RADIAL DISTANCE FROM DATUM 21.001                                             1         0.55815      -1.68003 -33.94285                                     2         -0.23160     0.72118  38.85700                                      3         -0.34418     1.02886  0.38850                                       4         -0.36503     1.20260  0.07000                                       5         -0.24853     1.24703  0.38850                                       6         -0.15947     1.12854  3.90285                                       7         0.08463      0.48930  18.85714                                      8         0.37931      -0.69051 0.                                            9         0.59104      -1.67175 0.01697                                       10        0.55815      -1.68003                                               ______________________________________                                    

It will be appreciated that having defined the profile of the bucket atvarious selected heights from the root, properties of the bucket such asmaximum and minimum moments of inertia, the area of the bucket at eachsection, the twist, torsional stiffness, shear centers and vane widthcan be ascertained.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A bucket for a steam turbine having a profile inaccordance with Table I inclusive as set forth in the specification. 2.A plurality of buckets, each constructed in accordance with claim 1, andspaced circumferentially about an axis of a turbine wheel, said bucketshaving tips, and covers continuously coupling said tips one to the otherabout said axis.
 3. A plurality of buckets according to claim 2 whereineach said cover includes a first tenon projecting toward an admissionside of the turbine wheel and a tenon projecting toward the dischargeside of the turbine wheel, the tips of adjacent buckets having openingsfor receiving the tenons for coupling the tips one to the other.
 4. Aplurality of buckets according to claim 3 wherein each tenon on a bucketadmission side is peened over to provide a rigid connection with anadjoining bucket tip and the tenon on a bucket discharge side has anopening in its end enabling the tenon to be flared, affording a looseconnection between the cover and adjoining bucket.
 5. A plurality ofbuckets according to claim 3 including a continuous sealing ribextending radially outwardly on the surface of each bucket tip and saidcover.
 6. A plurality of buckets, each constructed in accordance withclaim 1, and spaced circumferentially about an axis of a turbine wheel,said buckets having intermediate portions, each intermediate portion ofeach bucket including a nub projecting in a generally circumferentialdirection toward an adjacent bucket, a sleeve open at opposite endsreceiving the nubs of adjacent buckets affording a continuous couplingat the intermediate portions of the buckets.
 7. A plurality of bucketsaccording to claim 6 wherein each sleeve has an axis, the ends of thesleeves being formed at an angle to the axis other than 90° to permitthe sleeves to rock as the relative positions of the adjacent nubschange during operation of the turbine.
 8. A plurality of bucketsaccording to claim 6 wherein each nub has a non-circular cross-sectionand the sleeve has a generally corresponding non-circular cross-sectionto preclude sleeve rotation during turbine operation.
 9. A plurality ofbuckets according to claim 6 wherein the buckets have tips and means forcontinuously coupling said tips one to the other about the turbine axis.